How do I add something like this to the food database?

That's almost impossible if they don't post calorie information.....I would try to find out what size tortillas they use, 9", 10" etc and then look for something in the MFP database that matches the size and the flavour....get as accurate as possible....as for the filling, you can watch carefully how much they put on the tortilla and I'm afraid that you will just have to make your best guess.....this is why I don't eat at places that don't post calorie information....it's just too stressful.....best of luck!!!